1. Field of the Invention
The invention relates to wire machinery and, more specifically, to an apparatus for and method of differential length metering for the cabling of insulated conductors with fillers using the double twist process.
2. Description of the Prior Art
Machines, sometimes denominated as stranders, twisters, single and double twist twinners, quadders, single and double twist stranders, cablers and bunchers, have been in existence for many years. These machines are used to combine a plurality of individual wires and bunch or strand them together by imparting a single or double twist to them.
Typically, the individual strands or wires are payed off from a plurality of bobbins and directed at one input end of the machine or at both ends of the machine in the case of bunchers as described in U.S. patent application Ser. No. 602,667, assigned to the assignee of the subject application.
The wires are grouped or bunched together at the closing point prior to the entry into the machine. The closing point remains fixed relative to the main part of the machine.
The bunched wires or strands are then introduced into one end of a bow which rotates about the longitudinal axis of the machine. In the case of the double twist bunchers, it is the rotation of the bow that imparts a first twist to the wires at the input end of the bow while passing a first input pulley or sheave. Leaving the bow at the other end, the bunched and now single twisted wires pass over a second exit pulley or sheave which rotates with the bow. From this rotating sheave the bunched or stranded cable is directed over a sheave that is mounted on a cradle that is stationary in relation to the frame of the machine.
A second twist is imparted to the wire between the last sheave mounted on the bow and the sheave attached to the cradle. Additional pulleys disposed within the space defined by the rotating bow guide the now double twisted cable or wires to the bobbin supported within the stationary cradle and are wound on the bobbin itself while being evenly distributed thereon. Depending on the machine, slightly different wire guide systems have been used.
Double twist twinners, bunchers and closers have been extensively used in the electrical wire and cable, steel tire cord and steel rope industries for many years.
Typical machines are illustrated in the "Electrical Wire & Cable Machinery" catalog published by Ceeco Machinery Manufacturing Limited, the assignee of the subject application. Other exemplary structures of existing machines are disclosed in U.S. Pat. Nos. 3,570,234 and 3,732,682.
Machines for twisting a plurality of wires with the single twist system comprise a rotatable flyer and a reciprocally traversing reel rotatably supported within the flyer. A speed differential exists between the rotation of the flyer and the reel. A plurality of wires are fed from sources external to the machine, to the flyer for twisting the strands together. Due to the differential in rotation rates, the twisted strands are then wound from the flyer onto the reel.
In order to keep a constant lay, the rotation of the flyer and of the bobbin are controlled in such a way that a constant lay is maintained and a single twist is imparted to the individual wires fed through the flyer and onto the reel. Machines of this kind are described, for example, in U.S. Pat. Nos. 2,817,948 and 4,235,070.
The above machines are normally used to manufacture stranded or bunched conductors and to assemble two or more insulating conductors to form pairs, quads and other twisted conductors mainly used in the telephone communication industry.
Electrical cables, particularly those used in the telecommunications industry, are advantageously provided with one or more electrically conductive shields or screens which wrap around and enclose one or more groups of individual conductors. Such shields or screens help reduce pick-up of external electrical interferences, radiation and cross talk between adjacent conductors within the cable. The greater the conductivity of the shield or screen the better the results that are obtained. One form of shield or screen that is frequently used is a continuous tape coated at least on one side thereof with a conductive material. A metallized Mylar tape is commonly used. The tape can be helically wound or longitudinally applied about the conductor or conductors to be shielded or screened so that successive turns or lays of the tape overlap and make contact. For a tape made of a conductive material or a tape coated on both sides with a conductive material such overlapping contact provides the requisite conductivity of the shield. For tapes coated with a conductive material on one side only, the tape needs to be folded so that there is electrical continuity between successive turns or lays. In some instances a drain wire is wrapped on one or both sides of the tape shield to bridge successive turns and provide or enhance the required conductivity. Numerous cable designs have been proposed, each normally for a specific or particular purpose. Some examples of shielded cables which use tape to provide the shield or screen are described in the following U.S. Pat. Nos. 4,323,721; 4,327,246; and 4,406,914.
Taped conductors or assemblies have been traditionally made on single twist machines since the tape would be cracked or unacceptably stretched during the second twist imparted by a double twist machine.
Therefore, up to now the production of tape and/or screened products widely used in the telecommunication and specialty cable industries were made on slower machines. The same has been true for products with fillers as well.
The attempts to utilize double twist equipment was not successful because, as mentioned, the second twist imparted on the products at the end of the bow, would damage the taped conductor or assembly, thus producing cables of unacceptable quality.
Therefore, the state-of-the-art equipment can produce acceptable product only at slower speeds on single twist or equivalent machines.
In U.S. Pat. No. 4,574,571 an apparatus and method are described for manufacturing taped products with double twist equipment. This patent discloses the use of a pre-twister to meter in the correct amount of tape while taping using the double twist process. The disclosed apparatus works well for taped products. However, in some cables fillers are used in the production of insulated conductors in an effort to keep the outer surface of the cable as smooth or "round" as possible. The difficulty in producing these types of products using the double twist process results from the greater length of filler material required versus the length of conductor material required. This is due to the different cable centroid radii as suggested by R.sub.c and R.sub.f in FIG. 1. As a result, in a conventional double twist process, there is normally an inadequate amount of filler on the second twist. When stretched, the filler does not remain in the outer interstices but is drawn towards the center of the construction to thereby result in depressions in the outer surface of the cable instead of providing the desired smooth or "round" surface.
The use of a pre-twister alone to meter in the correct length of filler is not adequate, due to the integrity of construction (filler material is not rigid). The rotation of the product alone does not pull in the filler material.
The present invention, which overcomes this problem, consists of a metering wheel by which the conductors are wound one full wrap on the wheel in a groove of a specified diameter. The filler material is then wrapped on the same wheel but in a groove of a larger diameter. For each revolution of the wheel, more filler material is metered than conductor material, providing the extra length of filler material to avoid undue stretching when passed through a double twist machine. This results in a higher quality more uniform cable.